1. Field of the Invention
The present invention relates to an optical pickup suitable for recording an information signal on an optical storage medium (hereinafter, referred to simply to "an optical disc") or reproducing a recorded information signal therefrom, and an optical disc device using the same.
2. Description of the Related Art
In recent years, various optical pickups have been disclosed of a type adopting a diffraction grating, particularly, a holographic element for a detecting system, as a means effective to miniaturize and simply the optical pickup. For example, an optical pickup described in Japanese Patent Laid-open No Hei 8-77578 includes a holographic element disposed between an objective lens and a laser diode, and a four-division type photo-detector disposed in the vicinity of the laser diode, whereby a focus error signal is detected by a so-called spot size detection method and a tracking error signal is detected by a so-called push-pull method.
An optical pickup described in Japanese Patent Laid-open No. Hei 1-98131 includes a two-division type holographic element disposed between an objective lens and a laser diode, and a four-division type photo-detector disposed in the vicinity of the laser diode, whereby a focus error signal is detected by a so-called double knife edge method. In this related art document, there is also disclosed a photo-detector in which a division line of the photo-detector is tilted with respect to the diffraction direction of a reflected light of a diffraction element in order that light quantities of convergent light spots on optical detection areas on both sides of the division line are substantially equal to each other even if the convergent light spots on the photo-detector are displaced due to a variation in wavelength of a laser beam.
Optical discs generally used at present are mainly classified into two types from the viewpoint of the structure. One type is a so-called rewritable disc which allows an information signal to be recorded or erased along a continuous guide groove previously provided in an information recording plane of the optical disc. Another type is a so-called read only optical disc which only allows an information signal to be reproduced from irregular pit rows previously formed on the optical disc in accordance with information signals. Further, there are known optical discs of a type in which a rewritable area and a read only area are provided in one optical disc. In these circumstances, it is desired that the same optical disc device can freely, not exclusively, perform recording or reproducing for a plurality of kinds of optical discs different in disc structure.
However, in general, the push-pull method which is a tracking error signal detecting method most suitable for a rewritable disc having a guide groove is not suitable for a read only disc having no continuous guide groove, and reversely, a three-spot method which is most common as a tracking error signal detecting method for a read only disc cannot be applied to a rewritable disc. In other words, to perform recording or reproducing for a rewritable disc and a read only disc, there are required different tracking error signal detectors suitable for respective structures of the optical discs.
With respect to such a problem, there has been generally used an optical pickup capable of detecting only one of tracking error signals in accordance with a tracking error signal detecting method specialized for a rewritable disc using the push-pull method and a tracking error signal detecting method specialized for a read only disc, and there has been not known any optical pickup capable of detecting a plurality of kinds of tracking error signals by the same optical pickup.
An object of the present invention is to provide an optical pickup capable of including a tracking error signal detector specialized for a rewritable disc and a tracking error signal detector suitable for a read only disc by making use of a diffraction grating.
Another object of the present invention is to provide an optical pickup capable of reducing the size and simplifying the structure by making use of a diffraction grating.
A further object of the present invention is to provide an optical pickup capable of including a tracking error signal detector specialized for a rewritable disc and a tracking error signal detector specialized for a read only disc, and also enhancing a light availability by using a holographic element as a diffraction grating.